Electronics Production | November 23, 2004

White LED Driver for LCD backlight

STMicroelectronics has introduced a special part to drive the white LEDs that are becoming popular for color LCD backlighting or portable devices.
Color LCDs demand a pure white backlight so their colors are properly displayed. White LEDs are becoming the preferred technology for this but need a typical forward voltage of between 3V DC and 4V DC. The latest lithium-ion batteries have minimum operating voltages down to 2.8V DC. Since the brightness of an LED is proportional to its forward current, dedicated current-source power supplies are needed to drive white LEDs from the batteries used in portable applications.

The STLD20D is an inductive boost converter designed to drive a white LED with high efficiency over a wide range of battery voltages. It can drive up to four LEDs in series from a 2.8V DC battery voltage and offers dynamic supply voltage rejection to prevent backlight flickering. Conversion efficiency is more than 80%, according to STMicroelectronics.

The part includes an LED disconnect switch to reduce current leakage in stand by mode, soft start functions and a shut-down mode with dimming control. The STLD20D offers a low standby current of less than 1?A, and includes over-voltage and over-temperature protection.

The part has a pulse-width modulation input, so that a white LED can be dimmed without reducing its biasing voltage, which could affect the color of its output. This allows a pure white color to be kept over the entire dimming range.


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